If you are familiar with Mastermind, then you know how to play number mastermind. In place of colors, there are numbers.
Players try to deduce the hidden 3 or 4-digit code. You present a guess to your opponent and he/she indicates how many numbers are correct and in the right place and how many are correct and not in the right location.
You continue with subsequent guesses until you run out of turns or until the numbers are decoded.